Many types of sensors are available for the analysis of fluids. However, only certain sensors are designed to operate in real time, which is an essential prerequisite for the management of the combustion efficiency of an engine. Patent document U.S. Pat. No. 6,842,234 describes a sensor of this type comprising a two-strand optical fiber, one end of which is inserted as a probe into the liquid to be analyzed, and the other end of which is connected to a signal processing device.
The signal processing device generates an infrared optical signal in the first strand of said fiber, which terminates in the liquid opposite a reflector. This reflector reflects the optical signal towards the second strand of fiber. Accordingly, at the other end of the fiber, the processing device analyzes an optical signal which has passed through the probe which is immersed in the liquid to be analyzed. The temperature and composition of the liquid to be analyzed will modify the return optical signal to the processing device.
In order to ensure that the optical signal is fully reflected within the fiber-optic strands, two conditions must be fulfilled: the refractive index of the fiber-optic glass must be higher than that of the liquid through which the fiber passes, and the radius of curvature of the optical fiber must be sufficiently large. Accordingly, a sensor of this type is bulky in construction and costly.
Examples of less bulky and less expensive sensors are described in patent document U.S. Pat. No. 7,339,657. This document describes sensors based upon the principle of reflective infrared spectroscopy. According to this principle, an infrared ray is emitted by light-emitting diodes (LEDs) on the interior of an optical structure of the multi-faceted crystal type, the external facets of which are in contact with the liquid to be analyzed, and the internal volume of which accommodates the passage of the light ray.
This faceted structure ensures that there will be at least two reflections of the light ray in its internal volume. The outward path of the light ray from the LED and the return path of this ray to its point of analysis run in parallel, but in opposite directions. This outward/return path of the ray allows the infrared emitter and the analytical device to be installed on a single circuit board. Reflections of the infrared ray from the internal walls of the optical structure, upon the analysis of the return ray, allow the deduction of various physical or chemical parameters of the analyzed liquid on the other side of the optical structure. The structure of these sensors—incorporating LEDs, a single circuit board and a limited volume—is consistent with relatively low production costs and the on-board installation of said sensors in a vehicle.
However, the optical structure creates a projection on the interior of the volume of liquid to be analyzed. As a result of this projection, the circulating liquid forms a retention pocket ahead of the optical structure (in the direction of flow), such that residues (soot, impurities, etc.) will accumulate in said retention pocket. Moreover, the crystals used are both expensive and fragile.
The invention is intended to overcome these drawbacks by the application of transmission infrared spectroscopy and the use of a device for the continuous analysis of the total volume of fuel passing through an unobstructed duct, thereby permitting the analysis of the fuel flowing in said duct with no formation of a retention pocket in the liquid, and no resulting stagnation of residues.